AZ31鎂合金之硝酸鈰化成處理

方思凱(Ssu-Kai Fang)

指導教授 ： 林招松

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2004年)

https://doi.org/10.6342/NTU.2004.02504

摘要

本研究以AZ31鎂合金板材為基材，針對稀土鹽類硝酸鈰化成系統進行化成機構的建立，在不同化成溫度下，以硝酸調整pH值，並添加過氧化氫，觀察不同的製程條件對化成皮膜的影響。
實驗結果顯示，化成皮膜的生長首先是鎂離子的溶出，氫離子還原形成氫氣，試片界面局部pH值迅速上升，溶液中金屬離子以氫氧化物沉積，形成化成皮膜。在未添加過氧化氫的化成系統中，化成皮膜利用橫截面TEM試片製作進行微結構分析，皮膜共有三層，靠近底材的多孔層，接著是緻密層，外層為髮狀層。SEM觀察表面形貌，化成皮膜產生脫水裂紋，隨著化成時間增加，脫水裂紋變寬。並且皮膜容易剝落，顯示皮膜附著性不佳。
以硝酸調低pH值會限制皮膜生長，使皮膜厚度減小。添加過氧化氫，化成液由澄清透明轉變為橙黃色，化成液中三價鈰離子轉變成為四價鈰離子，所生成之皮膜為二氧化鈰，並且由於氫氣泡破裂所造成的震波，導致皮膜嚴重剝落，由橫截面TEM觀察顯示，為雙層結構。靠近底材為氫氧化鎂，外層為二氧化鈰與氫氧化鎂之混和物。增加化成系統溫度，可加速反應速率，增加質傳擴散的臨界距離，使皮膜增厚。

關鍵字

稀土金屬 ； 化成處理 ； 鎂合金 ； 硝酸鈰

並列摘要

Cerium conversion coatings were made on AZ31 magnesium plates. In addition to the basic cerium nitrate solution, the effects of solution temperature, pH and the content of hydrogen peroxide on the formation and microstructure of the coatings were also investigated.
Experimental results indicate that upon immersing in the solution, AZ31 plates dissolved to form magnesium ions and trace of aluminum ions. Meanwhile, the pH at the interface rose because of hydrogen liberation. This increase in interfacial pH caused the precipitation of magnesium and aluminum hydroxides. The coating formed in the cerium nitrate solution was characterized by TEM to consist of three layers with a porous layer overlaying the magnesium substrate, an intermediate compact layer followed by a fibrous layer as the major overlay. SEM observations showed that numerous cracks formed on the coating due to desorption of water molecules. Furthermore, the openings of the cracks increased with increasing immersion time, and more badly, might lead to the peel-off of the coating.
Reducing the solution pH by adding nitric acid inhibited the growth of the coating. Adding hydrogen peroxide to the clear cerium nitrate solution resulted in an orange solution, indicating that Ce3+ was oxidized to Ce4+. The coating formed in the solution with the addition of hydrogen peroxide comprised primarily the cerium oxide as the major overlay and a porous magnesium hydroxide layer intimately contacted with the substrate. Partial detachment of the cerium oxides from the substrate was observed and was ascribed to liberation of hydrogen bubbles. Finally, the growth rate of the coating increased with increasing solution temperature by means of enhanced mass transportation.

並列關鍵字

Rare earth metal ； conversion coatings ； magnesium alloys

參考文獻

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